The innate immune responses activate the adaptive immune responses, which work together to destroy infections (Figure 24-1). Unlike innate immune responses, adaptive immune responses are extremely specific to the pathogen that triggered them. They can also offer long-term protection. For example, antibodies produced by B cells will fight off future attacks by the same pathogen.
Innate immunity is our first line of defense against pathogens that enter through physical barriers such as skin or mucous membranes. It is made up of several components including granulocytes (neutrophils, eosinophils, and basophils), macrophages, dendritic cells, natural killer (NK) cells, and lymphocytes (T and B cells). These cells work together to identify invaders and trigger appropriate responses. In addition to defending against foreign particles, bacteria, viruses, and parasites, the innate immune system plays a crucial role in identifying self vs non-self molecules and initiating an inflammatory response if it determines that host tissue is being damaged.
Adaptive immunity consists of two branches: cellular and humoral. The cellular branch includes T cell and NK cell functions. The humoral branch includes antibody production by B cells.
T cells play an important role in protecting individuals from disease caused by infections with microorganisms that cannot be killed by neutrophils or macrophages.
The adaptive immune response is more sophisticated than the innate immune response. When an antigen is detected, the adaptive immune system generates an army of immune cells that are particularly tailored to target that antigen. Adaptive immunity also has a "memory" that improves subsequent responses to a given antigen. In contrast, the innate immune system responds to many different types of pathogens with the same cell types - neutrophils, macrophages, and dendritic cells. It is this adaptability that makes the adaptive immune response so effective.
Physical, chemical, and cellular protections against pathogens comprise the innate immune response. The primary goal of the innate immune response is to inhibit the proliferation and migration of foreign infections throughout the body as soon as possible. This is accomplished by activating phagocytes (cells that kill bacteria) and producing inflammatory mediators.
The innate immune system acts immediately upon infection with no prior training or instruction from previous exposures. It provides a first line of defense against pathogens that cannot be easily avoided such as viruses or bacteria that live within a host cell. The innate immune system also produces inflammatory mediators that help destroy invading pathogens and repair any damage done by their presence.
Innate immunity is divided into two branches: non-specific defenses and specific defenses. Non-specific defenses include physical barriers such as skin and mucous membranes that prevent pathogen entry, and chemicals in blood that act as natural antibiotics. These defenses can protect healthy cells as well as infected cells so there is no discrimination made between self and non-self.
Specific defenses require recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs). PRRs are proteins or molecules on host cells that detect PAMPs associated with pathogens. They activate the innate immune response after binding to these pathogens.
Inflammation is critical in pathogen management and in determining the adaptive immune responses that follow. When exposed to a pathogen again, these "trained" innate immune cells exhibit different inflammatory responses. They are still able to respond to other pathogens or molecules that can not activate T cells, but they do so with reduced intensity.
Innate immunity is our first line of defense against invading pathogens. It is activated by specific signals from pathogen-associated molecular patterns (PAMPs) that are expressed on bacteria or viruses. Bacteria and viruses have evolved mechanisms to inhibit or suppress the activity of the innate immune response. When this occurs, it allows the bacteria or virus to grow more freely which increases their chances of survival.
The three major classes of immune cells that participate in the inflammatory process are macrophages, dendritic cells, and neutrophils. These special cells release chemicals that cause pain and swelling at the site of injury or infection. They also help remove dead cells and cellular debris that may contain remaining traces of the invading organism.
The human body has natural defenses against invaders, such as bacteria, viruses, and parasites. These defenses include components of the immune system that are not classically classified as either innate or adoptive.
Innate immunity is something that the body already has. Adaptive immunity develops as a result of exposure to a foreign substance. Once triggered against a specific antigen, immunity lasts for the rest of one's life. Innate immunity does not need an exposure to become activated; it is always present but can be suppressed during an infection.
Innate immunity consists of two components: physical barriers that prevent pathogens from entering our bodies (such as skin or mucous membranes) and cells that act directly on any invaders that do make it past these defenses. The cells involved in innate immunity include macrophages, dendritic cells, neutrophils, eosinophils, basophils, natural killer cells, and mast cells. These cells work together to fight off bacteria, viruses, parasites, and other foreign substances that could otherwise cause serious illness.
Adaptive immunity requires exposure to foreign substances called antigens to become activated. After activation, your immune system produces antibodies which hunt down and kill any future infections by binding to their antigens. Two types of antibodies are produced: neutralizing antibodies and memory B cells. Neutralizing antibodies attack the antigen itself while memory B cells produce these antibodies later if the antigen reappears.
Neutralizing antibodies are important because they help protect us from re-infection with the same virus or bacteria.
The immune system defends against infection by employing tiered defenses with increasing specificity. Pathogens are kept out of an organism by physical barriers. When these barriers are overcome, the innate immune system responds with a non-specific, rapid reaction. This initial defense is provided by phagocytes (cells of the immune system that destroy bacteria) and natural antibodies. Natural antibodies are proteins produced by the body's immune system without specific antigen stimulation. They can be found in blood plasma or secretions such as saliva or breast milk.
Antibodies are used by the immune system to identify foreign cells and substances for destruction. Antibodies are produced by B lymphocytes. After being exposed to a foreign material, B lymphocytes divide rapidly, producing large numbers of antibody molecules. These antibodies are then released into the blood stream or other tissues where they bind to the foreign material and mark it for removal by other cells of the immune system or by complement. Complement is another component of the immune system that aids in the identification and elimination of invaders. It can also trigger inflammatory reactions that help fight off infections.
Immune responses are divided into two categories: cellular and humoral. The humoral immune response is mediated by antibodies which interact with specific antigens expressed on the surface of pathogens. These antibodies opsonize the pathogen causing it to be recognized by macrophages and other immune cells.